#2 Confirm message delivery using IRCv3.2's echo-message extension

When supported by the server, http2irc should use the `echo-message` extension of IRCv3.2 to confirm the message delivery. This should be more reliable than periodic `PING`s sent by the client (which aren't part of the IRC spec at all!). Relevant links: * Extension: https://ircv3.net/specs/extensions/echo-message-3.2 * Capability negotiation (relevant for the implementation here are REQ and ACK/NAK): https://ircv3.net/specs/core/capability-negotiation The current way of sending PINGs should still be retained for old servers that don't support `echo-message`. Unfortunately, even hackint does not support `echo-message` at this time because it's running on [https://github.com/charybdis-ircd/charybdis](Charybdis)'s 3.5 branch while support for that extension was only [added in 4.0](https://github.com/charybdis-ircd/charybdis/blob/7b8e4c0967681c7ba91df4dce488b9913ed1df75/NEWS.md#charybdis-40). So this is on hold until there is broader IRCv3.2 support on networks to make this (non-trivial) implementation worthwhile.